1. Field of the Invention
The present invention relates in general to coating apparatus, and more particularly, to apparatus for the electron beam vacuum vapor deposition of alloys of complex chemistry.
2. Description of the Prior Art
It has become increasingly important, particularly in certain gas turbine engine applications, to provide protective coatings on selected components such as turbine blades. Typical of coating alloys in wide use in gas turbine engines are the so-called MCrAlY coating alloys described in U.S. Pats. Nos. 3,542,530; 3,676,085; 3,754,903 and 3,928,026. One commonly used technique for applying these coating alloys to turbine components is by vacuum vapor deposition in which the substrate is exposed to vapors emanating from a molten pool of the coating alloy in a suitable vacuum chamber. Vaporization from the surface of the molten pool is effected by impinging an electron beam onto the surface of the pool. However, where high energy electron beams are utilized, serious problems arise in the form of severe thermal gradients in the pool which have been found to contribute significantly to chemistry variations in the vapor cloud and the as-deposited coating. The problem is aggravated when MCrAlY coating alloys are vaporized as a result of the elemental constituents of the coating alloys having widely differing melting points and vapor pressures under the coating conditions. One attempt to minimize chemistry variations in the as-deposited coating is represented by the Greaves patent, U.S. Pat. No. 3,655,430. Others may also include U.S. Pat. Nos. 3,205,087; 3,590,777 and 3,639,151.